I do not remember where are stored the cover images for albums.
If your not using the databases then it is
Album Image
%ProgramData%\Team MediaPortal\MediaPortal\Thumbs\Music\Albums\%artist%-%album%L.jpg
Artist Image
%ProgramData%\Team MediaPortal\MediaPortal\Thumbs\Music\Artists\%artist%L.jpg
Fanart Image (Requires Fanart Handler)
%ProgramData%\Team MediaPortal\MediaPortal\thumbs\Skin FanArt\Scraper\Music\%artist% (*).jpg
%ProgramData%\Team MediaPortal\MediaPortal\thumbs\Skin FanArt\UserDef\Music\%artist%*.jpg